100% ACCURATE TERRAMETER WORKING PARFECT LIKE ABEM

100% ACCURATE TERRAMETER WORKING PARFECT LIKE ABEM

INTRODUCTION

Electrical resistivity method was developed in the early 1900’s and has become much widely used since the 1970’s due to the availability of computer to process and analyze its data. These techniques are used extensively in searching for suitable groundwater and also monitor types of groundwater pollution. Resistivity is a fundamental and diagnostic physical property that can be determined by a wide variety of techniques under electrical surveying. Some make use of naturally occurring fields within the ground while others require the introduction of artificially generated current into the ground. Any subsurface variation in conductivity changes the flow of current within the ground and thus affects the distribution of electrical potential (Zohdy et al., 1974; Ojo et al., 1990; Olorunfemi and Okhue, 1992).

The basic method is to pass the current into the ground by means of two current electrodes and measures the potential difference via the potential electrodes. This technique applies to engineering and hydrological findings to investigate the subsurface geology. The self-potential method makes use of natural currents flowing in the ground that are generated by electrochemical processes to locate shallow bodies of anomalous conductivity. While electrical resistivity is the reciprocal of conductivity, it is a measure of the resistance of a piece of material of given shape and size. It utilizes direct current or low frequency alternating current to determine the electrical properties of the subsurface (Kearey et al., 1988). Resistivity of a formation often depends on the resistivity of the contained electrolyte and it is inversely related to the porosity and degree of saturation of the formation. It can be seen that the resistivity of rock layers vary from formation to formation and also within any particular layer (Flathe, 1970).

Because the need for fresh water in most part of Africa and many developing nations is so great, sustainable water sources are desired as well as training of indigenous scientist with skills to drill, develop and maintain good fresh water. The striking contrast in physical properties between the overlying unconsolidated material, clay or sand and the hard bedrock makes geophysical methods particularly attractive as means of detecting depth to bedrock. A well-known electrical resistivity technique is much suited to determine depth to bedrock under this geological setting, (Telford et al., 1990). For electrical resistivity meter, equipments have been designed by several authors (Herman, 2001; Hornbach, 2004; Olowofela et al., 2005). Olowofela et al. (2005) designed a resistivity meter in the context of geophysical research, but with limitation to water prospecting. The present study is an improvement over the design by Olowofela et al. (2005); as this instrument gives an increasing signal to noise radio.

MATHEMATICAL BACKGROUND By superposition, we include in the potential the irregular part from the point charge of electrode. Hence, for 0<Z<L: (1) the constant aλ has been absorbed in Aλ and Bλ . In matching boundary, it is most convenient to use the mathematical identity called Lipschitz integral identity: (2) which is valid for z>0. Hence, Eq. 1 can be written as: (3) (4) In order to solve the Laplace’s (or Poison’s) equation uniquely, appropriate boundary conditions must be applied. The first boundary condition to be satisfied is: (5) which implies that at the interface between the conductance (i.e., layer 1) and insulator (i.e., air above the ground), there is no accumulation of charges (or normal current flow between an insulator and conductor is zero). The second and third conditions to be satisfied are related to potentials and the potential gradient at the interface of two different conductors. The potential in layer 1 is φ1 and that of layer is φ2. At the interface where z = h, the two potentials must be equal i.e., (6) and for the potential gradient at the interface of two different conductors: (7) Applying equation (5) in Eq. 3 yields: (8) Applying Eq. 6 in Eq. 3 yields: (9) and (10) respectively. Using Eq. 8 in Eq. 9 gives: (11) this leads to: (12) From Eq. 10 we obtain (using Eq. 8): (13) Therefore: (14) Let us define: (15) Equating Eq. 12 to Eq. 14 and using Eq. 15, we obtain (16) where, K obeys the inequality -1≤K≤1. Substituting Eq. 16 in Eq. 3we obtain: (17) where: (18) For the four equally spaced electrodes and with two layers Earth, the potential difference between the two middle electrodes follows from the superposition application of Eq. 17 written conveniently as Eq. 10: (19) Hence: (20) Apparent resistivity ρa is given as: (21) FIELD PROCEDURE For the field operation, the equipment is placed midway between the potential electrodes using connecting cables to the potential terminals. The current electrodes are also connected to the current terminals. The potential and the current electrodes are usually made up of stainless steel metal rods of about 80 cm each. The cables on the electrodes arrangements are carefully done to avoid leakage and creep, which substantially reduce the attainable accuracy, sensitivity as well as the depth of penetration. Several methods have used for interpreting resistivity data and such includes partial curve matching technique (Zohdy, 1989; Telford et al., 1990), numerical analysis and Computer assisted techniques. RESULT The VES (vertical electrical sounding) shown the apparent resistivity values obtained for fabricated resistivity meter range from 543.67-2359.53 Ωm (Table 1) while that of the ABEM 300 B Terrameter range from 517.14-2333.35 Ωm (Table 2). And this reveals a fairly good degree of correlation. Table 3 shows the correlation between the results obtained when the fabricated equipment was used at the same location with the ABEM Terrameter 3000 B at the same location in a basement complex terrain of south western Nigeria. The electrode arrangement adopted was Table 1: Apparent resistivity using the fabricated resistivity equipment Table 2: Apparent resistivity using the ABEM 300 B terrameter Table 3: Comparison of apparent resistivity values using both instruments Schlumberger array but just for a shallow depth of current electrode separation of 12.0 m while potential electrode separation was 0.5 m. CONCLUSION From the results of both instruments, we can conclude that the instrument fabricated showed some degree of accuracy when compared to the ABEM 300 B common resistivity meter but still requires some improvement especially when subjected to deeper depth of investigation. This research work also dealt with some levels of theory in measuring the resistivity values of the Earth media.

REFERENCES

Flathe, H., 1970. Interpretation of Geoelectrical Resistivity Measurements for Solving Hydrogeological Problems. In: Mining and Groundwater Geophysics, Morley, L.W. (Ed.). Geological survey of Canada, Dept. of Energy, Mines and Resources, Canada, pp: 580-597. Herman, R., 2001. An introduction to electrical resistivity in geophysics. Am. J. Phys., 69: 943-952. CrossRef  |  Direct Link  |   Hornbach, M.J., 2004. Development and implementation of a portable low cost seismic data acquisition system for classroom experiments and independent studies. J. Geosci. Educ., 52: 386-390. Kearey, P., M. Brooks and I. Hill, 1988. An Introduction to Geophysics Exploration. 3rd Edn., Blackwell Science, Letchworth, UK., pp: 198-223. Ojo, J.S., T.E. Ayangbesan and M.O. Olurunfemi, 1990. Geophysical survey of a dam site: A case study. J. Mar. Geol., 26: 201-206. Olorunfemi, M.O. and E.J. Okhue, 1992. Hyrogeologic and geologic significance of a geoelectric survey at Ile-Ife, Nigeria. J. Mining Geol., 28: 242-250. Olowofela, J.A., V.O. Jolaosho and B.S. Badmus, 2005. Measuring the electrical resistivity of the earth using a fabricated resistivity meter. Eur. J. Phys., 26: 501-515. CrossRef  |  Direct Link  |   Telford, W.M., L.P. Geldart, R.A. Sheriff and D.A. Keys, 1990. Applied Geophysics. Cambridge University Press, Cambridge, Pages:761. Zohdy, A.A.R., 1989. A new method for ter automatic interpretation of schlumberger and wenner sounding curves. Geophysics, 54: 245-253. CrossRef  |  Direct Link  |

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